High power laser system

ABSTRACT

Disclosed is a system for decreasing the required angular velocity of a mechanically rotated Q switch for a laser for any given speed of switching the laser cavity from a low Q switch state thereof to a high Q state thereof. This is accomplished by incorporating means for providing a predetermined plural number of reflections by the rotating Q switch for each single round trip of the laser beam through the cavity.

United States Patent 1 Sims et al. ..33l/94.5

Nolan 1 Apr. 3, 1973 [54] HIGH POWER LASER SYSTEM 3,434,073 3/1969Forkner .331 945 [75] Inventor: Thomas E. Nolan, Medfield, Mass. I IAssignee: RCA Corporation Trion, Total Internal Reflecting Geometry Ruby[22] Filed; Sept 2, 1964 Rods Technical Bulletin T-l26l-l Dec. 4, 1961,l

pa e. [21] Appl. No.: 393,831 g Primary Examiner-Ronald L. Wibert 52U.S. Cl ..331/94.5, 350/285 [51] Int. Cl 58 Field of Search 331/945;350/6, 7, 285 [57] ABSTRACT Disclosed is a system for decreasing therequired an- [5 6] References Cited gular velocity of a mechanicallyrotated Q switch for a laser for any given speed of switching the lasercavity UNITED STATES PATENTS from a low Q switch state thereof to a high0 state 1,979,296 11/1934 thereof. This is accomplished by incorporatingmeans 2,670,660 3/1954 for providing a predetermined plural number ofreflec- 2,758,502 8/1956 tions by the rotating 0 switch for each singleround 3,315,177 4/1967 trip of the laserbeam through the cavity.3,328,112 6/1967 3,398,379 8/1968 7 Claims, 7 Drawing FiguresPATENTEDAPR3 1m 725 8 1 7 SHEET 2 [IF 2 INVENTOR, 77/04 145 5 Nam/v BYZ4: Affornea HIGH POWER LASER SYSTEM This invention relates to lasersystems and particularly to an improved optical system for use in alaser to obtain high power.

Many of the potential uses of lasers require a high power laser beam.One technique for obtaining high power in lasers is known as Qswitching. Q switching is based on the relationship between populationinversion threshold in a laser material, i.e. the minimum populationinversion required for laser action, and cavity Q. According to thistechnique a high population inversion below the threshold is establishedin the negative temperature laser material while the Q of the cavity ismaintained at a low value. After this population inversion isestablished the Q is rapidly increased thereby decreasing the thresholdvalue of population inversion and producing laser action. The result isa very intense laser beam.

One Q switching technique is based upon the characteristic of lasercavities that the cavity Q is determined in part by the degree ofparallelism between two reflectors or mirrors, one at either end of thecavity. One of the reflectors is made partially transparent to providean output beam. These two reflectors form a Fabry- Perot interferometerand are employed in lasers to provide a resonant structure for theoptical energy emitted by the active material. When the two reflectorsare parallel, then substantially no energy emitted in a directionperpendicular to the reflectors is lost, except that contained in theoutput beam. Thus when the reflectors are parallel, the effective Q ishigh. As the reflectors become less parallel, the effective Q of thecavity decreases.

One of the two reflectors may be maintained stationary while the otheris rotated about an axis parallel to its plane of reflection.Parallelism therefore occurs once per revolution of the rotatingreflector. When the two reflectors are parallel the cavity 0 is high;when they are non-parallel the Q is low. The active material is pumpedto a high population inversion during nonparallelism of the tworeflectors by synchronizing the firing of the laser pumping source withthe position of the-rotating reflector.

In the above described technique the transition from very low Q to veryhigh Q is not instantaneous but rather depends upon the angular velocityof the beam reflected from the spinning reflector and therefore on thespeed of rotation of the spinning reflector. As the spinning reflectorbegins to come into parallelism with the rotating reflector, the Q ofthe cavity begins to increase until the reflectors are parallel and theQ is maximum. Highest energy levels of the output beam are obtained by avery rapid switching from low to high Q. When the spinning reflectortechnique is employed, this means that very high rotational speedsshould be used. Limitations in the size and mechanical strength ofrotating parts which can be effectively employed to rotate thereflectors place limitations upon rotational speeds and therefore on theamount of power practicably obtainable by this technique.

It is therefore an object of the present invention to provide animproved optical system for switching the Q of a laser cavity.

It is a further object of the present invention to provide an opticalsystem for switching the Q of a laser cavity at higher rates than thoseheretofore practicably obtainable.

It is a further object of the present invention to provide improvedapparatus and techniques for obtaining relatively higher power levelsfrom lasers than those heretofore practicably obtainable.

It is a further object of the present invention to 'provide improvedapparatus and techniques for effectively multiplying the rateQ-switching of a laser cavity.

The above objects are accomplished according to the present invention byemploying in a laser system an optical multiplying system to increasethe effective rotational speed of a spinning reflector. Briefly, theoptical multiplying system of the present invention includes a rotatingreflecting surface and means to reflect a light beam a plurality oftimes from the rotating surface. As will be shown more fully below, theplurality of reflections cause the beam reflected by the last reflectionto move at an angular velocity which is greater than the angularvelocity when only the rotating reflector is used.

In one embodiment of the present invention the optical multiplyingsystem comprises two reflecting elements, (1) a rotating reflector,which may be a spinning flat or preferably a rotating prism, and 2) astationary reflecting surface positioned to reflect a light beam whichhas been reflected from the rotating reflector back to the rotatingreflector. A second stationary FIG. 2 is a diagram of a prior art laserQ switching arrangement; v

FIG. 3 is a diagram of a Q switching arrangement employing the presentinvention;

FIGS. 4 through 6 are isometric views illustrating three embodiments ofthe present invention; and

FIG. 7 is an isometric view illustrating an alternative embodiment ofthe present invention.

Before describing the embodiments of the present invention shown in thedrawing, a brief description of laser operation in relation to Qswitching will be given.

In general a laser includes at least three elements, (1) an active ornegative temperature material, capable of stimulated emission ofradiation, (2) a pump for supplying energy to the negative temperaturematerial and (3) a cavity containing the negative temperature materialwhich permits oscillation at the frequency of the output laser beam.

The essential characteristic of the negative temperature material whichmakes it suitable for laser operation is that a population inversion maybe established between two atomic energy levels. In the operationof thelaser, the population inversion is established between two energy levelscorresponding to the laser output frequency by causing low energyelectrons to be transferred to higher energy levels. This electrontransfer is caused by supplying input energy to the negative temperaturematerial from the pump. When the high energy electrons in the moredensely populated energy level return to the less densely populatedlower energy level, the device emits light at a frequency correspondingto the energy difference between the two states. The intensity of thelight thus produced is directly related to thev magnitude of thepopulation inversion.

In order for the light produced by the transfer to take the form of astimulated emission of radiation i.e. coherent laser radiation, theinverted population density, caused by pumping electrons from low energylevels to the higher levels, must be above a minimum or thresholdpopulation inversion. Population inversions below this threshold willnot permit laser action. The value of this threshold populationinversion is determined in part by the Q of the cavity containing theactive material. The laser cavity is constructed to permit oscillationsat the frequency of the output energy. Therefore the cavity stores someof the energy emitted by the negative temperature material. The Q of acavity is a measure of its ability to contain or store this energy andto prevent losses. As the losses of the cavity increase and the Qbecomes lower, higher values of population inversion are required forlaser action; i.e. the threshold increases. Therefore an active materialcan be pumped to a high population inversion without lasing if thecavity Q is maintained at a low value. In Q switching the laser materialis pumped to a high population inversion while the cavity is maintainedat a low Q. The Q is then switched to a high value and a very intenselaser beam is produced. v

FIG. 1 shows a laser Q switching arrangement employing the presentinvention. An active laser material 10, e.g. a ruby crystal, ispositioned between a partially reflecting surface 11 and a rotatableright triangular prism 12. A stationary reflector 13 isplaced near therotatable prism 12. The partially reflecting surface 11 forms aFabry-Perot interferometer with the stationary reflector 13. Therotatably prism 12 is rotated about an axis 20 parallel to the threetriangular sides of the prism by a motor 14 through a mechanicalconnection indicated by the dotted line 18. Input energy is supplied tothe laser material by a conventional pump 17. Where the laser material10 is a ruby crystal, the pump may take the form of thewell-known xenondischarge lamp. A source 16 controls the firing of the pump 17. Asynchronizing mechanism is employed to synchronize the flring of thepump 17 with the shaft position of the motor 14. I

For a discussion of prior techniques as well as a brief discussion ofthe present invention see Benson and Mirarchi, The Spinning ReflectorTechnique for Ruby Laser Pulse Control, IEEE Transactions on MilitaryElectronics, January 1964, pages 1 3-2]. If the spinning flat techniquewere used in the system of FIG. 1 then the flat would be rotated aboutthe axis 20 with its plane of reflection parallel to the axis. When theangular position of the flat were such that'its plane of reflection wereparallel to the partially reflecting surface 11, then substantially allof the energy, i.e.light, emitted from the end 22 of the laser material10 would be reflected from the flat back'into the laser material 10.This would be the high Q position. As the angular position of the flatdeviated slightly from this parallel posi tion the Q would decreaseuntil eventually none of'the energy emitted from the end 21 of the lasermaterial 10 would be reflected back into the laser material. Thisposition would correspond to zero Q. In the embodiment of the presentinvention shown in FIG. I the spinning flat of the prior art is replacedby a rotating prism 12 and a stationary reflector 13. The use of thesetwo elements accomplishes a much faster Q switching than was heretoforeavailable at moderate rotational speeds. The position of the rotatingprism 12 shown in FIG. 1 is the high Q position. Thus, a light beam 19,emitted from the end 22 of the laser material 10 is reflected from theprisms longest side 21 to the reflecting surface 13 and then backthrough the prism 12 to the laser material 10. The faces 23 and 24 ofthe prism are preferably coated with a suitable low reflectance coatingmaterial such as magnesium fluoride. A comparison of the multiplyingoperation of the rotating prism 12 and the stationary reflector 13 withprior art arrangements will be given below with reference to FIGS. 2 and3 after a brief description of the operation of the laser of FIG. 1.

The operation of the laser shown in FIG. 1 is essentially the same asthose which have been employed with the prior art techniques with theexception that the actualQ switching technique has been improved byincorporating the present invention. The laser material 10 is pumped toa high population inversion once per revolution of the prism 12 bysupplying energy to the pump 17 from the source For best operation thehigh population inversion should be established just before the prism 12reaches the high Q position.

Synchronization between the position of the prismv l2 and the operationof the pump 17 is therefore desirable. Conventional techniques may beemployed to obtain such synchronization. The synchronizing mechanism, 15may, for example, include a shaft position transducerwhich is arrangedto trigger the pump source 16 at the proper instant of time. Theparticular timing relationship between shaft position and operation ofthe pump source is best determined empirically for each individualsystem. As a general figure the prism 12 should reach the position shownabout 500 milliseconds after the pump 17 has been fired. The reflector13 is here shown as a flat. Generally it is advisable to use a Porroprism rather than a totally reflecting. flat in order to avoid alignmentproblems which may be present when a conventional flat is used.

FIGS. 2 and 3 show, respectively, diagrams of a conventional Q switchingtechnique and one embodiment of the present invention.

In FIG. 2 a portion 30 of a laser rod, such as the laser rod 10'of FIG.1, is shown. The end 31 of the laser rod 30 corresponds to the end 22 ofthe laser rod 10 in FIG. 1 opposite the partially reflecting surface 11.A reflecting surface 32 which generally takes the form of an opticalflat or a total internal reflection prism rotates about an axis 33parallel to its plane of reflection. The high Q position of the rotatingreflector 32 is indicated 'by the dotted line 34. That is, in theposition indicated by the dotted line 34, substantially all of theenergy in the form of light which leaves the laser material 30 from theend 31 is reflected back into the laser material 30. The position 'ofthe reflector 32 as shown in FIG. 2, i.e. A0 degrees from the high Qposition 34 is the position at which the Q begins to increase from zeroas the reflector 32 rotates counter-clockwise. In this position a lightray 40 emitted from one side of the laser material 30 is reflected fromthe surface 32 to the opposite side of the laser 30. Clearly, energyemitted from any other position of the end surface 32 is not reflectedback to the surface 31 but rather is reflected outside the end surface31. Thus, at this angular position A0, the Q of the cavity begins toincrease from zero until it reaches its maximum value when the reflector32 reaches the dotted line position 34. It is well known that maximumenergy outputs are obtained when the Q of the cavity is rapidly changedfrom zero to its maximum value. Thus, in the arrangement shown in FIG. 2the actual switching of the cavity Q from zero to its maximum valueoccurs while the reflector 32 is being rotated through the angle A0,.The time required for the change is therefore Mi /m where a), is theangular velocity of the reflector 32. In order to obtain extremely rapidswitching the rotational speed of the reflector 32 must be high. It isone purpose of the present invention to provide a Q switchingarrangement which provides extremely rapid switching at relatively lowrotational speeds.

The principles of the present invention will be described with referenceto FIG. 3. In FIG. 3 a reflector 32 is again mounted to rotate about anaxis 33 parallel to its plane of reflection and to reflect the lightenergy emerging from the laser material 30 at the surface 31. Thereflector 32 is here shown as a plane reflecting surface for purposes ofsimplicity in the following discus sion. Generally it is advisable toemploy a right triangular prism as shown in FIG. 1. An additionalreflecting surface 35 is located to the side of the reflecting surface32. The high Q position of the reflector 32 in the present embodiment isindicated by the dotted line 34. When the reflector 32 reaches thisposition substantially all of the light emerging from the laser 30 atsurface 31 is reflected back into the laser 30 by first being reflectedby the reflector 32 to the reflector 35 and then back to the reflector32 and back to the surface 31. The position of the reflector 32 in FIG.3 corresponds to substantially the same Q as does the position of thereflector 32' in FIG. 2. That is, when the reflector 32 in FIG. 3 is inthe position shown, the cavity Q just begins to change from zero to itsmaximum value. Thus a light ray 50 emerging from one side of the laserrod 30 is reflected via the path 50, 51, 52, 53 to the opposite side ofthe laser rod 30. The change from this position of the reflector 32 tothe high Q position indicated by dotted line 34 requires the reflectingsurface 32 to rotate through an angle A0,. Note that the angle A0, ismuch smaller than the angle A0. shown in FIG. 2. For equal angularspeeds, w, (0,, the change from low Q to high Q will be much faster inthe arrangement of FIG. 3 than that of FIG. 2 by a factor of 2.Therefore the arrangement of FIG. 3 provides a fast switching techniquefor moderate rotational speeds. In the technique of the presentinvention the light emerging from the laser rod reflects from therotating reflector twice, whereas in the prior art arrangement only onereflection is experienced. This double reflection causes an increasedangular velocity of the reflected beam over the angular velocity of thereflected beam 41 in FIG. 1. Greater increases in the switching rate maybe accomplished by causing reflections from the rotating reflector. Sometechniques for causing more reflections from the rotating reflector aredescribed below.

FIGS. 4 through 6 are diagrams of three embodiments of the presentinvention in which like reference numerals denote like elements. In FIG.4 an active laser material 60, e.g. a ruby crystal, is positionedbetween an optical flat 61 which is dielectrically coated for partialtransmission and partial reflection and a right triangular prism 62. Atotally internal reflecting prism 63 is positioned in proximity to theright triangular prism 62.

The non-reflecting surfaces of the two prisms 62 and 63 are preferablycoated with a suitable material such as a magnesium fluoride to preventlosses. The right triangular prism 62 is mounted to rotate about an axis64 which passes through the prism 62 and is parallel to and equallyspaced from the two equal length short sides 67 and 68 of the prism 62.At one angular position of rotation about the axis 64 the prism 62establishes a reflection coincidence between the optical flat 61 and thetotal internal reflection prism 63. The term reflection coincidence isused herein to describe a condition where a light ray reflected from afirst reflector along the same path by which it was incident upon thereflector, e.g. reflected perpendicularly from an optical flat, isreflected in the same manner from a second reflector and returns to thefirst reflector along the same path by which it was first incident uponthe first reflector. Thus in FIGS. 2 and 3 the dotted line 34 representsaposition at which reflection coincidence occurs. In FIG. 2 thereflection coincidence occurs between reflector 32 and the partiallyreflecting surface (not shown) at the other end of the laser rod 30. InFIG. 3 the reflection coincidence occurs between the reflector 35 andthe partially reflecting surface. In the reflection coincidence positionshown in FIG. 4 optical energy in the form of a beam 65 emitted from thelaser crystal passes perpendicularly through one of the two short sides67 of the prism 62. The light beam is reflected by the third side 69 ofthe prism 62 and passes perpendicularly out through the second shortside 68. The beam of light 66 which passes perpendicularly through theshort side 68 is incident upon the total internal reflection prism 63and is reflected back into the prism 62 along substantially the samepath by which it left the prism 62. The reflection coincidence positionor high Q position is established once per revolution of the prism 62.As the prism 62 rotates about the axis 64 the Q of the laser cavityperiodically changes from a zero value to a very high value. The angularvelocity of the reflected beam from the position 65 shown isapproximately twice that which would be obtained by employing a singlerotating flat. The time required for switching between the low Q valueand the high Q value in the embodiment shown in FIG. 4 is substantiallythe same as that of FIG. 3 described above.

In both of the embodiments of FIGS. 5 and 6 the light beam emerging fromthe laser material reflects a plurality of times from the rotatingreflector. As was pointed out above, as the number of reflections fromthe rotating surface increases, the angular velocity of the beam lastreflected increases and therefore the switching time decreases.

In FIG. 5, the laser crystal 60 is positioned between a partiallytransmitting flat 61 and a right triangular prism 70. A second righttriangular prism 71 is positioned to receive a light beam 75 reflectedfrom the prism 70 and to reflect the beam back into the first prism 70.A total internal reflection prism 72 is positioned directly under thelaser crystal 60. The right triangular prism 70 is rotatable about theaxis 73. As noted above, in order to prevent energy losses thereflecting surfaces of the prisms should be coated with a suitablematerial. In the position shown reflection coincidence occurs betweenthe flat 61 and the total internal reflection prism 72. A light beam 74emerging from the crystal 60 enters the right triangular prism 70perpendicularly through one of its short sides 77. The beam 74 isreflected by the side 79 perpendicularly out through the second shortside 78. The reflected beam 75 enters the right triangular prism 71 andis reflected back into the prism 70 along a path parallel to but spacedfrom the beam 75 along which it travelled to the prism 71. The beam isthen again reflected by the side 79 of the prism 70 and passesperpendicularly through the short side 77. The reflected beam 76 is thenincident upon the total internal reflection prism 72. The prism 72reflects the light beam back into the prism 70 and eventually back tothe laser crystal 60. In this embodiment of the invention, the angularvelocity of the last reflected beam and therefore the switching speedfrom a zero Q position to a high Q position is approximately twice thatof the embodiment shown in FIG. 4.

In general the switching speed may be increased to any desired value byincreasing the number of reflections from the rotating reflectingsurfaces. A further extension is shown in FIG. 6. Here the light beamemerging from the laser crystal 60 reflects a total of six times fromthe rotating surface 80 of the right triangular prism 81. In thisembodiment, two stationary right triangular prisms 82 and 83 arepositioned to reflect the light beam back into the rotating prism 81 tocause the multiple reflections. A total internal reflection prism 84 ispositioned to reflect the light beam back into the prism 81 alongsubstantially the same path by which it left the prism 81. In theembodiment of FIG. 6 the switching speed is three times of that of theembodiment of FIG. 4.

Although it is generally desirable to employ prisms to provide thenecessary reflections, any suitable reflecting surface capable ofwithstanding the intense energy of the laser beam may be employed. Thusin any of the embodiments of FIGS. 4 through 6 the rotating prisms maybe replaced by optical flats. Furthermore, while the total internalreflection Porro prisms 63, 72 and 84 of the three embodiments aregenerally desirable to avoid alignment problems totally reflecting flatsmay be employed in their place.

In the embodiments of FIGS. 4 through 6, the laser crystal 60 ispositioned between a partially reflecting, partially transmittingoptical flat 61 and the rotating prism of the Q switch. An alternatearrangement is shown in FIG. 7. In FIG. 7 a laser crystal 100 is cut atone end for total internal reflection. Two planar surfaces 101 and 102are cut at the end of the crystal 100 at an angle of 90 with respect toeach other. A partially reflecting, partially transmitting optical flat104 is employed to form the second reflector of the interferometer. Therotating prism 103 is placed so that at one angle of rotation about theaxis 105 reflection coincidence is established between'the totalreflection cut surfaces 101, 102 and the partially reflecting flat. Thearrangement of FIG. 7 is similar to that of FIG. 4 above in that thelaser beam is reflected only twice from the rotating prism 103. Any ofthe embodiments of FIGS. 4 through 6 above may be modified in the mannersuggested by the structure of FIG. 7.

What is claimed is:

1. In a laser comprising an active laser material within an opticalcavity for generating a beam of light from said active laser material,wherein said cavity is of the type incorporating a Q switch including amechanically. rotated reflective element for switching the Q of saidcavity between a relatively low value state thereof and a relativelyhigh value state thereof at a rate which is proportional to the angularvelocity of said rotated reflective element; the combination therewithof stationary additional reflective means forming part of said cavitywhich are oriented in cooperative relationship with said rotatedreflective element when said cavity is in its high Q state to cause saidbeam of light to be reflected by said rotated reflective element apredetermined plural number of times for each single round-trip pass ofsaid beam through said cavity, whereby the required angular velocity ofsaid rotated reflective element for any given switching rate is reducedin accordance with the value of said predetermined plural number.

2. The combination defined in claim 1, wherein said rotated reflectingelement is a substantially 45 right triangular prism rotated about anaxis substantially parallel to an edge of said prism, said beam fromsaid laser material being substantially normal to a first face of saidprism which includes one of its short sides when said cavity is in itshigh Q state, and wherein said stationary reflective means includesmeans oriented relative to said prism to reflect light emanating fromsaid prism in a direction normal to a second face thereof which includesthe other of its short sides when said cavity is in its high Q stateback in a direction normal to said second face.

3. The combination, defined in claim 1, wherein said predeterminedplural number is two.

4. The combination defined in claim 1, wherein said predetermined pluralnumber is four.

5. The combination defined in claim 1, wherein said predetermined pluralnumber is six.

6. In laser apparatus of the type including an active laser medium inthe form of a rod having opposite end face portions, one face portionbeing partially light reflective and the other face portion beingsubstantially totally light transmissive, means for pumping said lasermedium to effect emission of a light beam from said other face portion,rotatable reflector means for reflecting said beam of light back ontosaid other face portion when said reflector means is in one of itsrotated positions, and means for rotating said reflector means at apredetermined speed, the improvement comprising: optical means forreducing the duration of impingement of the reflected beam of light ofsaid other face to a value less than the duration of impingementcorresponding to the recited rotational speed of said reflector means,said optical means including fixed reflector means oriented with respectto said rotatable reflector means to effect a plurality of reflectionsof such reflected light to said rotatable reflector means, saidrotatable reflector means being oriented to reflect said light receivedfrom said first roof prism onto said second roof prism, said prisms inthe recited reflector orientation being effective to return said lightto said rotatable reflector means for reflection onto said other face ofsaid laser medium.

airm il

1. In a laser comprising an active laser material within an opticalcavity for generating a beam of light from said active laser material,wherein said cavity is of the type incorporating a Q switch including amechanically rotated reflective element for switching the Q of saidcavity between a relatively low value state thereof and a relativelyhigh value state thereof at a rate which is proportional to the angularvelocity of said rotated reflective element; the combination therewithof stationary additional reflective means forming part of said cavitywhich are oriented in cooperative relationship with said rotatedreflective element when said cavity is in its high Q state to cause saidbeam of light to be reflected by said rotated reflective element apredetermined plural number of times for each single round-trip pass ofsaid beam through said cavity, whereby the required angular velocity ofsaid rotated reflective element for any given switching rate is reducedin accordance with the value of said predetermined plural number.
 2. Thecombination defined in claim 1, wherein said rotated reflecting elementis a substantially 45* right triangular prism rotated about an axissubstantially parallel to an edge of said prism, said beam from saidlaser material being substantially normal to a first face of said prismwhich includes one of its short sides when said cavity is in its high Qstate, and wherein said stationary reflective means includes meansoriented relative to said prism to reflect light emanating from saidprism in a direction normal to a second face thereof which includes theother of its short sides when said cavity is in its high Q state back ina direction normal to said second face.
 3. The combination, defined inclaim 1, wherein said predetermined plural number is two.
 4. Thecombination defined in claim 1, wherein said predetermined plural numberis four.
 5. The combination defined in claim 1, wherein saidpredetermined plural number is six.
 6. In laser apparatus of the typeincluding an active laser medium In the form of a rod having oppositeend face portions, one face portion being partially light reflective andthe other face portion being substantially totally light transmissive,means for pumping said laser medium to effect emission of a light beamfrom said other face portion, rotatable reflector means for reflectingsaid beam of light back onto said other face portion when said reflectormeans is in one of its rotated positions, and means for rotating saidreflector means at a predetermined speed, the improvement comprising:optical means for reducing the duration of impingement of the reflectedbeam of light of said other face to a value less than the duration ofimpingement corresponding to the recited rotational speed of saidreflector means, said optical means including fixed reflector meansoriented with respect to said rotatable reflector means to effect aplurality of reflections of said beam of light by said rotatablereflector means prior to reflection of such beam by said rotatablereflector means onto said other face portion of said laser medium.
 7. Inlaser apparatus according to claim 6, wherein said optical meanscomprises at least a first stationary roof prism and a second stationaryroof prism, said first roof prism being positioned to receive lightreflected by said rotatable reflector means and effective to return suchreflected light to said rotatable reflector means, said rotatablereflector means being oriented to reflect said light received from saidfirst roof prism onto said second roof prism, said prisms in the recitedreflector orientation being effective to return said light to saidrotatable reflector means for reflection onto said other face of saidlaser medium.